在 ASP.NET MVC 应用程序中，我收到以下关于使用 Entity Framework 上下文的 Controller 方法之一的错误消息。

A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.

我知道您不能并行运行查询，而且一切似乎都在正确等待。如果我调试程序并逐步检查从 EF 返回的一些数据，那么它就可以工作，可能是因为这会强制完成查询。

编辑 如果我在 Controller 方法中的空检查处放置断点并检查 shipmentDetail 的数据，则不会抛出异常。

这是代码片段:

Controller 方法:

[Route("{id:int}/Deliveries")]
public async Task<ActionResult> DeliveryInfo(int id)
{
    var shipmentDetail = await db.ShipmentDetails.SingleOrDefaultAsync(s => s.Id == id);
    if (shipmentDetail == null)
        return HttpNotFound(string.Format("No shipment detail found with id {0}", id));
     var model = await DeliveryInfoModel.CreateModel(db, shipmentDetail);
    return View("DeliveryInfo", model);
}

CreateModel 方法:

public static async Task<DeliveryInfoModel> CreateModel(Context db, ShipmentDetail shipment)
{
    DeliveryInfoModel model = new DeliveryInfoModel()
    {
        ShipmentInfo = shipment
    };

    //initialize processing dictionary
    Dictionary<int, bool> boxesProcessed = new Dictionary<int, bool>();
    List<DeliveryBoxStatus> statuses = new List<DeliveryBoxStatus>();

     for (int i = 1; i <= shipment.BoxCount; i++ )
        {
            boxesProcessed.Add(i, false);
        }

        //work backwards through process

        //check for dispositions from this shipment
        if(shipment.Dispositions.Count > 0)
        {
             foreach (var d in shipment.Dispositions)
            {
                DeliveryBoxStatus status = new DeliveryBoxStatus()
                {
                    BoxNumber = d.BoxNumber,
                    LastUpdated = d.Date,
                    Status = d.Type.GetDescription().ToUpper()
                };

                statuses.Add(status);
                boxesProcessed[d.BoxNumber] = true;
            }
        }

        //return if all boxes have been accounted for
        if (boxesProcessed.Count(kv => kv.Value) == shipment.BoxCount)
        {
            model.BoxStatuses = statuses;
            return model;
        }

        //check for deliveries
        if(shipment.Job_Detail.Count > 0)
        {
            foreach (var j in shipment.Job_Detail.SelectMany(d => d.DeliveryInfos))
            {
                DeliveryBoxStatus status = new DeliveryBoxStatus()
                {  
                    BoxNumber = j.BoxNumber,
                    LastUpdated = j.Job_Detail.To_Client.GetValueOrDefault(),
                    Status = "DELIVERED"
                };

                statuses.Add(status);
                boxesProcessed[j.BoxNumber] = true;
            }
        }

    //check for items still in processing & where
    foreach (int boxNum in boxesProcessed.Where(kv => !kv.Value).Select(kv => kv.Key))
    {
       //THIS LINE THROWS THE EXCEPTION
        var processInfo = await db.Processes.Where(p => p.Jobs__.Equals(shipment.Job.Job__, StringComparison.InvariantCultureIgnoreCase) && p.Shipment == shipment.ShipmentNum && p.Box == boxNum)
                                .OrderByDescending(p => p.date)
                                .FirstOrDefaultAsync();

       //process returned data
       //...
    }

    model.BoxStatuses = statuses;

    return model;
}

我不完全确定这是因为在 Controller 中进行的查询，还是因为在循环中进行的查询未完成导致此行为。由于 EF 的懒惰，或者 async/await 在这种情况下如何工作，我是否不了解实际进行/返回查询的时间？我有很多其他方法和 Controller 可以进行异步 EF 调用，但以前没有遇到过这种情况。

编辑

我的上下文使用 Ninject 作为我的 IoC 容器注入(inject)到我的 Controller 中。这是它在 NinjectWebCommon 的 RegisterServices 方法中的配置:

kernel.Bind<Context>().ToSelf().InRequestScope();

最佳答案

在 Entity Framework 中使用异步时避免延迟加载。相反，要么首先加载您需要的数据，要么使用 Include() 来确保您需要的数据随查询一起加载。

https://msdn.microsoft.com/en-gb/magazine/dn802603.aspx

Current State of Async Support

... Async support was added to Entity Framework (in the EntityFramework NuGet package) in version 6. You do have to be careful to avoid lazy loading when working asynchronously, though, because lazy loading is always performed synchronously. ...

(强调我的)

还有:

https://entityframework.codeplex.com/wikipage?title=Task-based%20Asynchronous%20Pattern%20support%20in%20EF.#ThreadSafety

关于c# - 为什么 Await 不会出现阻止对 EF 上下文的第二次操作，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/34773533/